As is well known the combustors for gas turbine engines have, from a technical standpoint, made significant advances in the state of the art over recent years. Combustor efficiency, for example, for aircraft jet engines operates in the high 90 percentile. Additionally, the technology has made significant improvement in reducing or eliminating pollutants and/or smoke emanating from the combustion process.
However, the requirements for combustors that are intended to meet demands for future aircraft needs will require even further advances in combustor technology. Obviously, aircraft engine performance is predicated on attaining high turbine inlet temperatures. Higher inlet turbine temperature, within a given limit, will manifest into improved thrust-to-weight ratios and specific fuel consumption with a consequential improvement in engine performance.
Thus, future demands will require that the advanced combustion system attendant these high performance engines will have to operate at a temperature rise at high power that is significantly higher than that of state-of-the-art combustors. However, it is of paramount importance that the designer of the combustor meets the increased temperature rise requirement without degrading heretofore established levels of performance, smoke density and pollutants exhausting from the engine.
In addition to the demanding requirements already alluded to, the combustor must be able to be re-lighted within specified altitudes. And when these engines are employed in lightweight aircraft, it is contemplated that the combustor will operate at lower than the levels of temperature rise associated with state-of-the-art combustors during engine deceleration and idle and will be required to be sufficiently stable in order to facilitate ground handling.
To gain better insight to the complexity and operability problems posed by high temperature rise combustors, it perhaps will be worthwhile comparing the requirements of the combustor with the stability characteristics of the combustion process. To this end, the graph which is a plot of fuel/air ratio vs. the stability correlation parameter is presented in FIG. 1. As is known, since most combustion occurs in the combustor's primary zone or region, this section of the combustor is selected for discussion purposes. The stability correlation parameter which has become a standard measurement in the combustor art contains the following terms applied to the primary zone:
V=Average through-flow velocity PA0 P=Pressure level PA0 T=Inlet Air Temperature
When these terms are combined in a non-dimensional parameter (V/PT.sup.2) increasing velocity, decreasing pressure and/or decreasing temperature increases the stability parameter to higher levels. They likewise adversely affect the combustion process by making it more difficult for supporting combustion. Stated another way, higher values of the stability parameter incur more severe and more difficult combustor operating requirements.
As is noted from an inspection of FIG. 1, the curve defines stability limit which is generated by reducing and increasing fuel/air ratios until blowout occurs. Hence, combustor operations falling within the left-hand side of the parabola-shaped stability limit curve A will be stable and any operations to the right of curve A will be unstable. Further, curve B defines an upper limit of fuel/air ratio and operations above this limit will exhibit excessive levels of smoke.
Hence, as is apparent from the foregoing in combustor operation for conventional, state-of-the-art combustors, the primary zone fuel/air ratios are set so that they fall below curve B and the engine deceleration, idle and altitude re-ignition requirements fall within the left-hand side (stable operation) of curve A. This is shown by curve C and altitude re-light operation is defined as operating point E, using as a standard an altitude of 30 thousand feet and the aircraft flight speed of 0.8 Mach Number as the point of re-ignition.
Curve F represents the combustor operations at increased levels of temperature rise contemplated for future advanced technology engines. Obviously, to assure combustion operation is below the smoke limit (B), the combustor primary zone air flow must likewise be increased, moving the combustor operating curve F closer to stability curve A, which obviously illustrates the increased severity of the requirements of the combustor. And, as noted, the deceleration and the altitude ignition represented by point G may fall outside of the stability limits (curve A). And what is indeed apparent, the trend is such that as temperature rise increases to a higher value, the combustor's operating line moves even to more severe requirements such that even the idle (points H and J on curves C and F, respectively) may likewise fall outside of the stability limit (curve A).
Engineers and scientists have been battling with this problem for some time and have attempted to resolve it by several different approaches. Some of these approaches, all of which have exhibited significant disadvantages, include variable fuel staging, variable geometry and double annular combustor. There are no known solutions demonstrated in the prior art which satisfactorily solve the problems alluded to above.
Fuel staging contemplates delivering fuel to the combustor through fuel nozzles wherein some nozzles flow more fuel than others so that fuel flow is scheduled for two or more types of fuel nozzle. By proper scheduling it is therefore possible to provide localized fuel enrichment in a portion of the combustor thereby holding the deceleration and idle conditions of curve F within the stability limits as shown by the dash line K extending from the bottom of curve F. These systems do not resolve the basic stability requirements and may still have the altitude ignition problem as noted by point G' in the graph of FIG. 1.
Variable geometry combustor offers a more realistic approach to solving the problems enumerated above but only at the expense of additional costs, weight and complexity associated with the hardware necessary to make the geometry variable. In this approach, the airflow to the primary zone is varied by mechanically adjusting the air metering orifices. Hence, the fuel/air ratio and stability parameter at given combustor operating points are altered so that the operating requirements remain in the stable region which is to the left of curve A.
The last of the approaches suggested is the double annular combustor and this combustor arrangement utilizes airflow staging to solve the problems alluded to above. An example of a double annular combustor is disclosed in U.S. Pat. No. 3,934,409 granted to H. A. Quillevere et al on Jan. 27, 1976.
In these combustors the primary zone is made up with an inner annulus and an outer annulus. One of these annuli (primary) is designed to accept a relatively low airflow which will exhibit good stability characteristics. The other annulus (secondary) accepts a relatively higher airflow and is allowed to exceed stability limits at low power and altitude ignition operating conditions. Given that the primary annulus will sustain and propagate combustion to the secondary annulus, combustion will be sustained and the combustor is relightable at altitude conditions.
However, the double annular construction inherently requires extra air admission devices and fuel nozzles resulting in a heavier and more expensive combustion system.
These types of approaches described above as well as conventional state-of-the-art combustion systems all provide circumferentially uniform airflow distribution. In some instances the airflow distribution may be locally tailored in some manner around fuel nozzles or diffuser struts to provide uniform exit temperature distribution. An example of localized tailoring of airflow around the fuel nozzle is disclosed in U.S. Pat. No. 4,696,157 granted to G. Y. G. Barbier et al on Sep. 29, 1987. This patented system as well as the other systems alluded to herein repeat the airflow distribution around the combustor and this distribution is a function of the fuel nozzles or a function of diffuser struts in the case of attempting to compensate for the diffuser struts.
In all these instances, the designer intends to provide circumferentially uniform or circumferentially repetitive airflow distribution. Additionally, the fuel flow distribution is uniform from nozzle to nozzle during high power operation.
Another approach that is worthy of mention is the approach disclosed in U.S. Pat. No. 4,720,970 granted to D. A. Hudson et al. on Jan. 26, 1988 and that approach is akin to the variable geometry approach alluded to in the above. In the U.S. Pat. No. 4,720,970, supra, the dome of the annular combustor is circumferentially divided into a sector or sectors. Airflow control valves vary the airflow in a given sector(s) to define a region in the primary zone for varying the fuel/air ratio and control the burning in that sector(s). However, like the variable geometry approaches mentioned above, this system incurs the same disadvantages.
We have found that we can obviate the disadvantages detailed hereinabove while extending combustor lean extinction and stability limits.